The 12 volt systems used in today's automobiles are required to supply ever increasing currents as the load on the system continues to increase. This increase is due to a combination of increasing numbers of electronic devices, such as communication, entertainment, and telematics systems, as well as the proliferation of electric powered auxiliary systems to replace traditional hydraulic or mechanical powered systems. To reduce the amount of current required to supply these higher loads, it has been proposed that automobiles should adopt 42 volt electrical systems. However, the automotive industry has been reluctant to transition to 42 volt electrical systems because of increased costs. Consequently, there is a strong demand to improve the performance of 12 volt systems, thereby allowing higher electrical loads to operate effectively with conventional vehicle electrical systems.
As an example, high current loads, such as electric power steering (EPS), cannot practically be used in larger vehicles, such as light trucks, with conventional vehicle electric systems. EPS in particular places a large demand on the electrical system because it draws a large current at low vehicle speeds, which is where the most steering assist is required. However, at low vehicle speeds, e.g. in a parking lot, the engine is typically at or near idle and thus alternator current output capability is severely limited. As a result, the vehicle electrical system cannot supply the power needed by EPS without the 12 volt bus experiencing a temporary voltage dip. When this voltage dip occurs, a variety of objectionable performance is experienced from various electrical systems, for example dimming of the vehicle lights. Additionally, it is also likely that the required EPS current cannot be supplied, and thus the desired steering response will not occur.
A variety of solutions to the problem of supplying high current loads in vehicle electrical system have been proposed. European patent application EP0533037A1, entitled “An Electrical System for a Motor Vehicle, Including at Least One Supercap” describes a circuit and supercapacitor arrangement that is connected across a load. The load is energized initially from the supercapacitor. The amount of energy drawn from the supercapacitor is not optimized because the supercapacitor is connected directly across the load, thereby limiting the voltage drop across the supercapacitor. Also, there is no isolation of the load/supercapacitor circuit from the battery other than a simple diode, so the temporary power provided to the load is not entirely decoupled from the battery.
U.S. Pat. No. 5,914,542, entitled “Supercapacitor Charging” describes a DC power distribution system for a fighter aircraft. In the described system, the battery is located remotely from the load. A supercapacitor is connected to the DC bus through a supercapacitor boost converter combination close to the load. The supercapacitor is normally disconnected from the bus, but when a load transient occurs, the battery is disconnected from the load and the load is supplied from the supercapacitor. This system is disadvantageous in that the energy supplied to the load is limited solely to that in the supercapacitor.
The present invention attempts to minimize the above-mentioned drawbacks and proposes a system that solves or at least minimizes the problems of the prior art.
While the invention is susceptible to various modifications and alternative forms, specific embodiments have been shown by way of example in the drawings and will be described in detail herein. However, it should be understood that the invention is not intended to be limited to the particular forms disclosed. Rather, the invention is to cover all modifications, equivalents and alternatives falling within the spirit and scope of the invention as defined by the appended claims.